1. Field of the Invention
The present invention relates to a thyristor structure and to a monolithic assembly of thyristors having common cathodes and common gates, the gates being biased with respect to the cathodes.
2. Discussion of the Related Art
FIG. 1 schematically represents a conventional thyristor structure formed from an N-type substrate N2 having a thickness of several hundred microns. A P-type layer P2 is uniformly formed on the lower side of the substrate and corresponds to the thyristor anode, coated with an anode metallization A. On the side of the upper surface a P-type region P1 is formed, corresponding to a cathode gate layer in which a N-type cathode region N1 is formed. A metallization G is integral with the gate region; a metallization K is formed on region N1 and forms the cathode electrode. Interruptions of region N1 that correspond to conventional emitter shorts, are also represented.
At the periphery of the thyristor, from the upper and lower surfaces P-type, diffusions 2 and 3 are formed to provide a structure that is commonly referred to as a well thyristor structure.
In such a structure, it is the anode surface that can be mounted on a heat sink and that corresponds to the side of the cooling system of the thyristor and also, generally (except in insulated circuits), to the heat sink voltage. With such a structure, several thyristors having common anodes can be easily integrated in parallel.
In such thyristors, the gate is biased with respect to the cathode. That is, the thyristor becomes conductive if, while its anode is positive with respect to its cathode, a positive voltage is applied between its gate and its cathode to allow a current to flow from the gate to the cathode.
For a long time, manufacturers of thyristors have optimized the doping levels and the shapes as viewed from the top, of the various layers as well as the shape of the shorting holes. This was done in order to optimize the various desired parameters of such a cathode-gate thyristor, such as its breakdown voltage and its switching-on and switching-off parameters.
FIG. 2 illustrates some of these parameters.
If the anode-cathode voltage is a positive voltage equal to V1 and if a gate voltage is applied, as shown by curve 10, the anode-cathode current rapidly increases. Then, the voltage drops to a low value until the voltage and the current are established at V2, I2, which correspond to the parameters of the circuit that the thyristor is inserted into. Then, the thyristor remains in this state even if the gate current is interrupted. The thyristor is said to be sensitive to switching-on if a low gate-cathode voltage and a small flow of current in the gate are sufficient to trigger the on state.
To turn off the thyristor, the voltage across its terminals must decrease until the current flowing in the thyristor becomes lower than a hold current IH.
Additionally, the thyristor has a given forward breakdown voltage; that is, if in the absence of a gate current the voltage exceeds a threshold value V.sub.BR, the thyristor breaks down and the voltage-current characteristic curve corresponds to curve 11. In addition, this thyristor breakdown voltage depends on the rapidity of the voltage surge, this characteristic corresponds to the dV/dt sensitivity of the thyristor.
The above parameters of a thyristor (switching-on sensitivity, dV/dt sensitivity and hold current) are of primary importance. However, these various parameters are contradictory. More particularly, by decreasing the rate of emitter shorts, the sensitivity to the switching on is increased, whereas by increasing this rate, the problems associated with dV/dt triggering are decreased, and hold current IH is increased.
However, with structures including thyristors having a common anode and a gate that is biased with respect to the cathode, as illustrated in FIG. 1, it has been possible to satisfactorily optimize all these parameters. It is known that it is possible to further increase the switching-on sensitivity by providing cathode-gate amplification thyristors (also called darlistors).
In some applications, it is desirable to obtain thyristors in which the gate electrode is on the side of the anode metallization so that the cathode metallization is alone on a surface of the component and can be directly mounted on a heat sink. It would be possible to use anode-gate thyristors. However, the fabrication of such thyristors raises problems that are presently not well solved.
Indeed, it has generally been asserted in patents that an equivalent structure to a cathode-gate thyristor could be obtained by inverting all the conductivity types of N-type and P-type layers. However, despite these statements, there is in fact no equivalence between an N-type layer and a P-type layer. More particularly, the mobility of the carriers is different in an N-type layer than in a P-type layer, and it is not possible to obtain doping levels in N-type layers equal to doping levels in P-type layers. For example, it is difficult to obtain very highly doped levels for P-type layers.
The solution that would consist in transforming the thyristor shown in FIG. 1 into an anode gate thyristor by inverting all the conductivity types is not satisfactory, especially due to the fact that it would be impossible to substitute a very highly doped P-type layer for layer N1.
Another solution for providing an anode-gate thyristor is illustrated in FIG. 3 in which layer P1 including the cathode-gate is changed into an anode layer P2. The substrate N2 in this case operates as an anode gate. Layer P1 becomes an unconnected cathode-gate layer, and the cathode layer N1 is formed on the side of the lower surface. Such an arrangement provides a rather little unsensitive structure due to the fact that the gate is connected to a very thick layer N2 (the substrate) that is the layer that determines the breakdown voltage.
Thus, in practice, cathode-gate thyristors, such as the thyristor illustrated in FIG. 1, are predominantly available, and there are practically no anode-gate thyristors. Therefore, it is simply possible to form circuits in which several parallel thyristors have a common anode that is connected to a heat sink. However, it is very difficult to connect in parallel thyristors having a common cathode.
A further drawback of anode-gate thyristors is that, even if it were possible to fabricate efficient anode-gate thyristors, they inherently would have a gate biased with respect to their anode, whereas, in many electrical circuits, it is desirable to bias the gate with respect to the cathode and not to the anode.